The danger comes from the other ingredients in the pills such as magnesium stearate and corn starch which are very bad for you to deposit in a vein to say the least, people used to inject tablets of a narcotic called diconal because it apparently makes heroin look like codeine
the problem is these tabs had silicone in them which got shot up with everything else and caused all kinds of problems, some even had to have legs and arms amputated.
There are ingredients which are safe as well, in fact if you type "codeine" by itself in Rxlist you get a monograph for tablets made purely of codeine, lactose and sucrose made specifically for injection
http://www.rxlist.com/cgi/generic/codphos.htm
these will work just fine but only because the sugars are completely harmless and soluble. Dilaudid brand name tabs are another that are composed of the sugar glucose and are injectable. Generally if its a pill its been made to stay together all the way into your stomache with all kinds of binders. some are soluble with the drug and some arent, some will make a huge mess when you add water to them, others may work perfectly.
The general rule is dont try and inject pills because for the most part the binder isnt good for you at all and may cause serious problems, for the few that are injectable which are made of sugars, you may inject theses and infact the pharmaceutical community makes tablets specifically for it so its not just a street trick its valid and normal, you just need a good pill. The other problem is that even though a pill may be injectable it may not be the same as on the monograph and may be different such as Dilaudid brand names and the generics where they are composed differently and the generics are worth less because you cant inject them. Im leaving you with a magic link its called Rxlist and they have the ingredients for just about every pharmaceutical on the first page of the monograph just type the drug.
http://www.rxlist.com/
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